Due to the upcoming release for the DLC this week, Valve has posted the achievements that we'll be able to earn.
There are a few standard ones, but others have to do with new things.

Standard ones:

Torch Bearer
Survive The Passing Campaign.

Port of Scavenge
Play 5 full games of Scavenge on The Port.

Fore!
Knock off the heads of 18 infected with the golf club.

Til It Goes Click
Using the M60, kill 25 infected without letting go of the trigger.

Killing Them Swiftly To This Song
Play the new Midnight Riders song on a jukebox.

Wedding Crasher
As the Charger, grab a Survivor and crash them through 8 chairs at the wedding.

So the standard achievements make us know that there will be a map with a wedding that took place, the weapons etc. were already known.

These are the ones that can be seen as spoilers (including the new game modes):

Spoiler

Cache Grab
Open 5 foot lockers.

Kite Like a Man
Kill a tank only with damage from the original Survivors.

Grave Robber
Collect 10 items dropped by a Fallen Survivor.

Mutant Overlord
Play 6 Mutations.

We also know that the three of the four original survivors will be unplayable bots (I guess) that protect us.
The foot lockers also are completely new, it most likely has items in it like weapons and medical things.
The Fallen Survivor was already known.

Quote:

Originally Posted by Kotaku

Valve also reveals "Mutations," featuring more than 20 new game modes rotated on a weekly basis, from "Realism Versus" to "Chainsaw Massacre."

April 20, 2010 - Microsoft has confirmed that the first downloadable expansion for Left 4 Dead 2, The Passing, will be available on Xbox Live this Thursday, April 22 for 560 Microsoft Points.

The Passing introduces new gameplay features, most notably the "uncommon common" infected type, as well as new weapons and an additional co-op mode. The campaign is playable in Survival, Versus and Scavenge modes.

Be sure to check out IGN's latest impressions of the expansion. No word yet on the release date for the PC version.

Really interesting news..I hope that the DLC will be released the same day on PC.

They're both coming out on the same day.

From Shacknews:

Quote:

[Update - 2:00pm CST] Valve has now confirmed that "The Passing" will hit both Xbox 360 and PC on Thursday, April 22. As expected, the Xbox 360 download will be priced at 560 Microsoft Points / $7, while those playing on PC will get it for free.

Great news. But I will assume like with Crash Course for L4D1 that Xbox users will get the DLC round the start of the day while us PC users will still be waiting for it until perhaps middle of the day I think.

Great news. But I will assume like with Crash Course for L4D1 that Xbox users will get the DLC round the start of the day while us PC users will still be waiting for it until perhaps middle of the day I think.

All depending on Valve and their good Steam.

Yeah I guess so, since Valve manages the Steam release and Microsoft the Xbox one.